ci: Auto-stop Review Apps after 6 hours

What does this MR do and why?

Given that number or resources is a problem for <code data-sourcepos="3:51-3:62">external-dns</code> and maybe also the cause for the nginx-controller issues (1 and 2), we'll now auto-sop Review apps after 6 hours instead of 48 hours.

That should give way enough time for end-to-end tests to run against the Review Apps and use the Review App manually. There's always the possibility to disable auto-stopping if someone's need the Review App to stay up longer.

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